1. Field of the Invention
The present invention relates generally to analog-to-digital converters, and more specifically, to a delta-sigma modulator-based analog-to-digital converter in which a dither value is introduced to the noise-shaping filter and then compensated by removal from the quantized converter result.
2. Background of the Invention
Delta-sigma modulators are in widespread use in analog-to-digital converters (ADCs) and digital-to-analog converters (DACs), in which they provide very linear behavior and simple implementation due to the reduced number of bits used in the analog signal comparison. Delta-sigma modulators can be implemented with a high level of control of the frequency distribution of “quantization noise”, which is the difference between the ideal output value of the modulator as determined by the input signal and the actual output of the modulator provided by a quantizer. The relative simplicity of the architecture and the ability to finely control the quantization noise makes delta-sigma converter implementations very desirable.
The delta-sigma modulator based ADC typically includes an analog noise-shaping filter that receives an input signal and a quantizer that converts the analog output of the noise-shaping filter to a digital representation. A feedback signal provided from the output of the quantizer is provided to the analog noise-shaping filter to close the loop such that the average output of the quantizer is equal to the value of the input signal. The output of the quantizer is then filtered by a low-pass digital filter having a large number of taps, in order to provide an accurate conversion result from the quantizer output, which typically includes hundreds of values per conversion cycle.
Dither is often introduced to the analog noise-shaping filter or directly into the quantizer of a delta-sigma modulator-based ADC, in order to ensure that the signal level being quantized has sufficient changes so that the quantizer output is constantly changing and therefore providing the above-described noise-shaping operation. However, in precision ADCs, the addition of dither generates an error in the output that is undesirable. Therefore, application of dither is a trade-off between providing sufficient dither to ensure that the converter produces no “stuck” codes, while minimizing the level of error due to the dither.
Therefore, it would be desirable to provide a delta-sigma modulator that has a reduced frequency of stuck code sequences, while producing little or no error due to the introduction of dither. It would further be desirable to provide such a delta-sigma modulator that can be reset for intermittent operation.